1. Field of the Invention
The present invention is in the fields of molecular biology and protein engineering. The invention is directed to methods for the production of recombinant proteins. More specifically, the invention is directed to methods for producing recombinant proteins as inclusion bodies in bacteria, particularly Escherichia coli. The present invention also provides plasmids, vectors and host cells to be used in the present invention for production of recombinant proteins, and methods of purification of the proteins produced by these methods. The invention is also directed to proteins produced by these methods. The invention is also directed to methods for production of protein molecular weight marker ladders, and to ladders produced by these methods.
2. Related Art
With the advent of recombinant DNA technology, it has become almost routine to produce large amounts of proteins in heterologous expression systems, such as transformed host cells, for commercial and basic research purposes. Among the expression host systems, E. coli is the most popular system because of ease with which E. coli can be manipulated. However, expression of heterologous proteins in host cells has some limitations. These include: inefficient translation of mRNA due to the presence of infrequently used codons (Kane, J., Current Opin. Biotech 6:494-500 (1995)), instability of mRNA in E. coli (Bachmair, A. et al., Science 234:179-186 (1986); Olins, P. & Lee, S., Current Opin. Biotech 6:501-506 (1993)), toxic effect of the protein being expressed (Brosius, J., Gene 27:161-172 (1984); Studier, W. & Mofatt, B., J. Mol Biol. 189:113-130 (1986)), and formation of inclusion bodies because of inappropriate folding of the protein (Schein, C., Bio/Technology 7:1141-1149 (1989); Mitraki, A. & King, J., Bio/Technology 7:690-697 (1989)). To solve these problems, a variety of techniques have been developed.
Gene fusion is one of the most popular strategies to express proteins of interest. This particular technique is used to produce large amounts of heterologous protein by fusing the protein of interest to the carboxy terminal end of a fusion partner (LaVallie, E., and McCoy, J., Curr. Opin. Biotech. 6:501-506). As an example of this approach, methods have been developed for selective isolation of a desired protein or polypeptide by constructing a recombinant vector containing a DNA sequence coding for the desired protein or polypeptide which is operatively linked to a DNA sequence coding for protein A (WO 84/03103). The expressed fusion protein is then selectively isolated by adsorption onto an IgG-supporting carrier, which binds protein A, followed by desorption of the fusion protein. The fusion protein is then cleaved at a unique cleavage site with a cleavage agent, which may include proteases, hydroxylamine, cyanogen bromide or formic acid, to give the purified protein.
Most systems used for the manufacture of recombinant polypeptides attempt to minimize the production of the polypeptide in inclusion bodies in the expressing host cells. One important reason for these attempts is that the production of polypeptides in inclusion bodies often yields a biochemically inactive, denatured, or otherwise functionally or structurally compromised polypeptide upon its release from the inclusion bodies via standard solubilization techniques. While a variety of methods have shown some promise in minimizing inclusion body formation, gene fusion techniques in particular have been utilized to produce soluble proteins which otherwise would have been produced as inclusion bodies.
However, the formation of inclusion bodies within host cells can also be advantageous. For example, inclusion bodies constitute highly dense and concentrated “packets” of the desired polypeptide, from which contaminating host cell proteins can be removed by methods as simple as centrifugation. After their isolation, controlled conversion of the inclusion bodies to a soluble form could provide a rich source of the desired polypeptide in its pure, biologically active or structurally intact form. The difficulty with such an approach, however, has been that it is usually nearly impossible to predict whether or not a recombinant polypeptide will form inclusion bodies when it is expressed in a host cell.
Thus, the current invention provides a system in which controlled formation of inclusion bodies is used to produce a desired polypeptide. By this controlled formation of inclusion bodies, purification of the desired polypeptide is rendered faster and more complete, and subsequent controlled solubilization of the inclusion bodies provides a high yield of pure polypeptide in its active form.